TUESDAY, Jan. 15 (HealthDay News) -- Flu vaccination rates among U.S. children were lower than expected over a recent five-year period, a new study reports.

The findings were released in the midst of the current flu season, with 47 states now reporting widespread illness, according to the U.S. Centers for Disease Control and Prevention. Health officials recommend that all children 6 months and older get the flu vaccine.

For the new study, Wake Forest Baptist Medical Center researchers reviewed data on more than 8,000 children younger than 5 in three counties in Ohio, New York and Tennessee between 2004 and 2009, and found that less than 45 percent of them received a flu shot.

"Our research showed that one in six children under age 5 who went to an emergency department or clinic with fever and respiratory symptoms during the peak flu seasons had the flu," study author Dr. Katherine Poehling, an associate professor of pediatrics and epidemiology, said in a Wake Forest news release.

"Many of those illnesses could have been prevented by vaccination, the best known protection against the flu," she added.

Children younger than 6 months of age had the highest hospitalization rates with flu, the study authors noted.

"Parents should include a yearly flu shot to protect themselves and their children," Poehling said in the news release. "The best way to protect infants too young to receive the influenza vaccine is for pregnant women, the infant's family members and contacts to get the shot, too," she added.

The study was published in the online edition of the February issue of the journal Pediatrics.